We went straight into OSFA from about 2-3wks and had no trouble with fit. We had Bum G, Haute and a Baby beehinds to start with.

The good thing about OSFA is that a lot of them are pockets and dry quicker than your AIO so a good choice if you want to minimise drying times however if you want to look at some OSFA AIO try cuddlesncloth abbygro and nappylicious and bum g make one also but I don't know how well it sits on a newborn as the booster folds up at the back of the nappy and looks a little uncomfy.

If your expecting a big bubba and you really want AIO go for the medium size 5-11kg as you will get more use out of them then a small (usually around 2.5 - 6-8kg)

As for how many we started with 5 plus some flats and I washed every night, but I had a dryer. Minimum I would recommend with no dryer is 14 but this is assuming your winters are reasonably dry. I would go for 22 to be safe and try lots of different brands, some pockets, some AIO so you can see which fit your bub the best. Some stores do a multipack with differnt brands: darlings downunder, nappybucket, nurture nappies, maybe baby blossom?